Sugar and Salt from Sugarcane

Sugarcane is the largest agricultural crop in the world in terms of tonnage and provides an annual impact on the Louisiana economy of nearly $3 billion. In 2016, Louisiana produced 1,612,000 tons of raw sugar, equivalent to more than one third of the domestic cane sugar production. This amount was produced from sugarcane planted on 431,000 acres. Louisiana sugarcane is processed in 11 raw sugar factories. The bottom line of the sugar industry ultimately depends on the crystallization of granular sugar (sucrose) from syrups. While these syrups contain typically more than 85 percent sucrose, complete recovery is not possible because natural impurities, such as other sugars (fructose, glucose) and inorganic compounds (ash), limit the recovery of crystalline sucrose. In particular, the inorganic ash hinders the complete recovery and is detrimental to the process. Typical ash levels in Louisiana range from 1.5 percent to 2.7 percent in syrups. When the raw sugar is crystallized from the syrup, the ash components remain in the molasses and reach a level of up to 15 percent. At this level, ash and other impurities limit the commercial recovery of sugar, and up to 10 percent of the initial sucrose in the syrup remains in blackstrap molasses.

The level of remaining sucrose is governed by the impurities in the blackstrap molasses, where a natural limit — the so-called “target purity” — exists. At this limit, the impurities — especially the ash —prevent the sucrose from crystallizing and, therefore, its recovery. Current industry efforts aim at optimizing their operations to get as close as possible to the target purity within their given impurity restrictions.

LSU AgCenter scientists at the Audubon Sugar Institute are investigating an alternative approach. Instead of solely trying to meet the target purity, their research aims at moving the target purity to lower levels to allow more sucrose to be recovered. The key element is the reduction of the ash content because an ash reduction by 25 percent in a typical raw sugar factory in Louisiana would allow the recovery of an additional $500,000 worth in sugar. While there are many technologies available, electrodialysis is being investigated as the most promising because it does not require chemicals and has greater flexibility when it comes to handling variable ash levels.

Electrodialyis is a membrane-based separation that uses electricity to move ions through ion exchange membranes out of the molasses or syrup into a separate stream (concentrate), thus de-ashing the syrup. Figure 1 shows the principle components of an electrodialysis cell, where membranes of alternating polarity are stacked between two electrodes, and the ions migrate based on their polarity to the individual electrode. The syrup is kept in one chamber and depletes in ions, resulting in de-ashed syrup. The ions concentrate in the other chamber, yielding a salt solution. As such, it is possible to de-ash syrups without using chemicals.

Initial tests indicate that up to 66 percent of the ash can be removed from a given stream, while sugar losses are within acceptable range. Typical energy consumption ranges up to 22.7 kilowatt-hours of electricity per ton of sugarcane syrup for a 66 percent de-ashing level. For the aforementioned 25 percent reduction in a sugar mill, an electric energy consumption of only 4.53 kilowatt-hours per ton of syrup is expected. The electricity cost of $75,000 per sugar season compares favorably with the potential benefit of increased sugar recovery valued at $500,000. Research is being conducted to improve the technology and to better estimate the cost.

While de-ashing with electrodialysis is in itself desirable because of increased sugar recovery, the process yields a salt solution as waste stream. Typically, the salts found in sugar syrups in Louisiana consist of 95 percent potassium, 2 percent calcium, 2 percent magnesium and 1 percent sodium and ammonium salts with chloride as the dominant anion. This salt solution is therefore particularly rich in potassium chloride, also known as muriate of potash. When this salt solution is concentrated through evaporation, muriate of potash crystallizes in high purity as a valuable side product. Muriate of potash can be either used directly as fertilizer or be further processed into chemicals such as potassium hydroxide or hydrochloric acid. The production of potassium chloride from sugarcane is a first for Louisiana and marks one of the few cases where an inorganic mineral is produced from a renewable, sustainable source.

The potential of this technology is promising because annual black strap molasses production in Louisiana reaches 407,000 tons, where an estimated amount of 61,154 tons of sulfated ash are present. If 25 percent of this molasses would be de-ashed, the Louisiana sugar industry would be able to produce 13,081 tons of muriate of potash (worth $2.5 million), supplementing the fossil-based mining operation with a novel, sustainable plant-based source.

Franz Ehrenhauser and Daira Aragon are assistant professors at the Audubon Sugar Institute.